A two-stroke engine typically includes a scavenging port that communicates with the crank chamber and opens out at a side wall of the cylinder bore so that a mixture containing fuel is supplied from the crank chamber to the cylinder bore via the scavenging port, and this flow displaces or scavenges the combustion gas remaining in the cylinder out of the combustion chamber at the same time. The scavenging port is opened and closed depending on the position of the piston that reciprocates in the cylinder bore such that the scavenging port communicates with the combustion chamber defined above the piston when the piston is near the bottom dead center, and is shut off from the combustion chamber when the piston is away from the bottom dead center.
In such a two-stroke engine, it is known to provide a flow guide such as louver fins adjacent to the scavenging openings at the cylinder bore wall in order to create a circumferential component in the scavenging flow. The circumferential component may also be varied by changing the angle of the flow guide. See JP63-183323U. By thus directing the scavenging flow in the circumferential direction, a swirl flow of the mixture is created in the cylinder bore.
However, this prior proposal is not configured to change the timing of opening and closing the scavenging port. If the timing of opening and closing the scavenging port can be varied, the volume of the mixture that is supplied to the combustion chamber and the amount of the internal EGR can be varied depending on the operating condition so that the output and efficiency properties of the engine can be improved over a wide operating range.